This invention relates in general to the field of wave soldering systems, and in particular to a method and apparatus for acoustic pressure assisted wave soldering.
Wave Soldering is an established method of soldering electronic components to a Printed Circuit Board (PCB). There is a wide variety of equipment and processes available to wave solder PCBs, however, the general principles of the process remain the same, as explained below.
Components are assembled to the PCB by placing component leads into plated through holes or by gluing surface mount components to a bottom surface of the PCB. This unsoldered assembly is called a Printed Circuit Assembly (PCA) or module with components loosely mounted or glued. Flux is applied to the module, and the flux removes the oxides from the surfaces to be soldered. The module is preheated to activate the flux and reduce thermal shock subjected by the wave of molten solder. A wave of molten solder is applied to the bottom side of the module. The molten solder fuses to the exposed metal surfaces of the module, which include annular rings, pads, and component leads. Additionally, capillary and wetting forces draw the solder into the plated through holes. The excess molten solder is removed from the bottom of the module typically by transferring the module up and away from the molten solder, allowing gravity and surface tension to separate the molten solder from the module.
The limitations of this process continue to be challenged as the inclusion of surface mount components on the bottom surface of the module, and the decrease in size of the line widths, spacing between adjacent vias, and distance between leads. Defects such as solder skips resulting from a lack of sufficient solder and solder bridges and icicles resulting from excess solder continue to create quality issues during the manufacturing of electronics products.
The desirable outcome of the wave solder process is for components to be sufficiently soldered to the module to form reliable, electrically conductive, and mechanically robust solder joints. Solder interconnects which comprise insufficient solder are sometimes called solder skips and are undesirable because they may cause open circuits. Solder interconnects which comprise excess solder are called solder bridges (between two adjacent solder connections) or icicles (excess solder hanging from a solder connection) and are undesirable because they may short contacts, leads or pads together, and potentially cause electrical failure. Various processes and devices are available to avoid these skips, bridges and icicles. In one case, reference is made to U.S. Pat. No. 5,240,169 where the module is soldered in a reduced oxygen atmosphere for reducing oxides on the solderable surfaces of the PCB and components improving wetting of the surfaces. The typical gas used in this method is nitrogen. The gas is continually fed into the soldering area to limit the amount of oxygen present. This method is expensive, as relatively pure nitrogen must be used. In another case, reference is made to U.S. Pat. No. 4,684,056, which discloses a method for vibratory wave solder, promoting solder penetration into narrow spaces between components, filling of small holes in a board, filling crevices, and all other areas where solder wetting is difficult to achieve with conventional machines. This method of mass vibration of the solder pot is difficult to control, sometimes causing too much solder to be present potentially raising components to a non-desirable position and causing excess solder or solder balls on the component side of the module, and sometimes too little, not allowing the system to be effective. Additionally, there is a potential to damage sensitive components with direct mechanical coupling of vibrational energy. In yet another case, reference is made to U.S. Pat. No. 5,228,614 where a non-oxidizing gas air knife is used to remove solder bridges and icicles. The air knife passes air across the assembly. Air flow increases heat transfer, as it removes the thermal boundary layer created between the module and the surrounding air temperature, similar to a common phenomena referred to as wind chill. This may cause undesirable thermal stresses on the solder joints. This method is also difficult to control for a wide variety of modules. Too much pressure causes solder droplets to fly out of vias, sometimes landing on the module. Too little pressure is ineffective. Additionally, the method requires the air knife to be in close proximity to the module, limiting clearance on the bottom of the module. Reference is also made to U.S. Pat. No. 3,921,888, which discloses the wave soldering of printed circuits wherein the pins projecting through a circuit board move on a conveyor at approximately the same speed as a smooth stream of solder from the solder wave so that the relative movement between the pins and the solder is substantially a vertical movement to avoid side icicles of solder forming. This method does not reduce the level of defects to an acceptable level and is usually combined with other defect reduction strategies.
While each of these improvements has contributed to the art, skips, bridges, and icicles remain a problem in the industry.
Thus, what is desirable, is a means to reliably connect components to form the desired solder interconnects to assemble a module while minimizing common defects such as skips, bridges and icicles.
The present invention addresses the deficiencies in the art by applying acoustic pressure waves proximate to the interface of the molten solder and the module such that the energy aids to improve the manufacturing yield. The pulsating energy increases the yield over the prior art solutions by breaking up any solder shorts or removing icicles, as well as causing small surface waves which eliminate solder skips.
One aspect of the current invention is to provide a means to apply solder to the desirable side of a module.
A second aspect of the present invention is to provide a means to use acoustic pressure waves to remove skips, bridges, and icicles from the module.
A third, aspect of the invention is to provide a means to apply flux to the module prior to providing a means to apply solder to the module. The flux is applied to remove the oxides on the solderable surfaces, resulting in improved solderability.
A fourth, aspect of the present invention is to provide a means to apply the acoustic pressure waves on the module to reduce bridges while utilizing other process improvement techniques known in the art, such as inert gas soldering, non-oxidizing air knife soldering, vibrational soldering from within the molten solder, and/or controlled solder velocity (non-turbulent wave) soldering.
The present invention comprises an apparatus used to apply molten solder to the bottom surface of a module to create electro-mechanical interconnects during the assembly of a module. The apparatus preferably includes equipment known in the art: a conveyor to move the module across a flux applicator, through preheaters, and across a molten solder bath.
The present invention discloses the application of acoustic pressure waves to add energy to the molten solder proximate along the final interface between the molten solder wave and the module, thus overcoming the surface tensional forces. The acoustic pressure waves, transferred to the boundary between the module and the molten solder bath, preferably is at the exit of the molten solder bath, and may create shearing forces between the module and the molten solder bath. These shearing forces may force the excess volume of molten solder adhering to the module to fall downward into the molten solder bath. Because the module is preferably above the molten solder, gravity may aid in the complete transfer of the excess volume from the module to the molten solder bath. The acoustic pressure wave energy applied to the interface creates small waves and aids in creating solder interconnects on joints which may otherwise have created a skip, while removing excess solder as described above.
The acoustic pressure waves may preferably be created by a transducer and transferred to the module and molten solder bath boundary through the air.